Toxoplasma gondii is an important opportunistic infection of AIDS patients. Improved strategies and approaches are urgently needed to more effectively prevent and treat recurrent infections in AIDS. Due to the ease of in vitro culture and cloning, efficiency of transformation, abundance of established genetic tools, and availability of robust murine models of infection, Toxoplasma gondii is increasingly recognized as a model apicomplexan parasite for understanding biology of intracellular parasitism and host immune response. Our laboratory has developed a highly attenuated uracil auxotroph (cps1-1) mutant of T. gondii (RH background) that is proving to be a valuable model organism for understanding critical host-parasite interactions. The cps1-1 mutant invades host cells, but does not replicate in vitro or in vivo in the absence of uracil. Tachyzoites of the uracil auxotroph cps1-1 can be cultured in vitro and following intraperitoneal (i.p.) inoculation into mice elicit potent Th1 responses that confer effective long-term protection from a lethal challenge infection from virulent type I strain RH. Our recent results demonstrate that cps1-1 administered i.p. elicits surprisingly effective long-term protective immunity from lethal primary and chronic infections after peroral challenge with type II ME49 cysts. Most commonly, the natural route of exposure and transmission of Toxoplasma to humans is via oral ingestion of tissue cysts. Bradyzoites released from cysts launch a primary infection in gut mucosa that ultimately leads to the establishment of chronic brain cysts that can later cause Toxoplasmic encephalitis in AIDS. The early host-parasite interactions during T. gondii infection in the gut mucosa are likely to be critical in eliciting innate and adaptive immune responses that lead to immune protection. We propose to develop tet regulated uracil auxotrophy in type I and type II strains with a goal to develop live-attenuated cyst-forming mutants that can be delivered by the natural peroral route. The strains developed in this project should provide valuable tools necessary for the dissection of key host- parasite interactions in early infection in gut mucosa and will also serve as innovative models for studying host response, parasite development, trafficking, cyst biology, immunity, and vaccines. Relevant to Toxoplasmic encephalitis infections in AIDS, the model organisms developed in this project are likely to provide invaluable tools for experimentally addressing cyst formation, cyst maintenance, and cyst recrudescence. This research project will develop and validate model organism strains of Toxoplasma gondii with the property of regulated virulence and development. These model organisms are expected to provide significant improvements in the ability to perform critical investigations into host response, parasite development, and host-parasite biology as these areas relate to new concepts for effective vaccination to prevent or treat brain cysts. Consequently, this research project will accelerate the discovery of new vaccines and treatments for devastating opportunistic parasite infections in AIDS.